Environmental Engineering Reference
In-Depth Information
the scale loses its protective ability. In such cases, reaction may be governed by
diffusion through (1) a thin reaction product layer of approximately constant
thickness adjacent to the metal or (2) by phase boundary reactions. The linear
oxidation rate commonly observed in alkali metals is due to this type of reaction
behavior. Depending on the system under investigation and the reaction condi-
tions, other processes may also take place. This may involve the formation of
reaction products that are liquid or that may continuously evaporate (MoO
3
,
CrO
3
, etc.).
In general, the oxide layer that forms on the metal during oxidation are found
to be compact, adherent, and protective if the criterion formulated by Pilling and
Bedworth (PB) [5] in 1923 is satisfied. The PB ratio is expressed as:
V
oxide
(molar volume of oxide)
V
metal
(volume of metal consumed for formation of 1 mol of oxide)
φ
(5.6)
In general, when this ratio is greater than unity, protective film or scale growth
is expected to occur on the corresponding substrate. However, if it is too much
greater or too much lower than unity, then the compactness of the scale is lost
and there will always be free access of gaseous reactant to the fresh metal surface.
The PB ratios of some common metals are listed in Table 5.1.
Although the rule has many exceptions, it is historically important and is still
used even today as a rough guideline in predicting the protective nature of the
compound layer formed. Schottky first pointed out the extent to which this vol-
ume quotient idea will play its role in maintaining adherence and compactness
of the scale, so that solid state diffusional transport remains the rate-limiting step.
Furthermore, it would be decided by the level of stresses developed in the oxide
and the underlying metal as well as by the extent of simultaneous stress relief
through plastic flow of the oxide and the substrate. Although attempts have been
made to rationalize the situation by considering all of these mechanical aspects,
it is really difficult to develop a unified theoretically sound background that will
be fully reliable to explain the behavior and abnormalities of various systems.
In this respect, knowledge of ratio of linear thermal expansion coefficients for
Table 5.1
Pilling-Bedworth (PB) ratios of some typical oxide-metal systems
K
2
O
i
2
O
MgO
CdO
Al
2
O
3
ZnO
Cu
2
O
NiO
FeO
0.45
0.58
0.81
1.21
1.28
1.55
1.64
1.65
1.68
TiO
2
CoO
Cr
2
O
3
SiO
2
Ta
2
O
5
Nb
2
O
5
U
3
O
8
MoO
3
WO
3
1.76
1.86
2.07
2.15
2.50
2.68
2.77
3.30
3.35
